Last night, James Cameron was in Silicon Valley and had a chat with Google CEO Eric Schmidt. During a Q&A session he said that he would like to shoot the sequels at a higher framerate, maybe 48 fps or more. Cameron already made a statement about that he wasn't really content with the 24 fps after the release of Avatar because of motions artifacts in fast scenes, so it is very likely that this will happen if it doesn't turn out to be too expensive. He also mentioned that they want to perfect water effects and light effects in the water which will be key elements of the sequel(s).
He also confirmed Avatar 2 and 3 as his next projects, talked about the difficulties concerning the training of the motion capturing system to get a good result from the capture (especially the faces) and about this and that concerning the future of 3D and his diving project for the sequels.